Yeah, I'm often one to criticise Google, but an entirely free cloud-based photo album was never going to be around forever. If you are very concerned with keeping an online record for all time you really shouldn't be depending on free stuff like Picasa.
Maybe, but it would also cost Google almost nothing to set up a 301 redirect from Picasa URLs to Google Photos. I wouldn't expect someone like Yahoo or Microsoft to bother, but being a “good web citizen” seems intrinsic to the Google corporate identity, after all.
But that doesn't solve the issue that the photos on Google Photos are private by default, so Google would need to change the sharing to public on all photos that have external links, which would be a terrible privacy measure.